Anti-backlash screw jack

ABSTRACT

An anti-backlash screw jack has a worm gear which is supported by upper and lower annular bearings both of which remain fully loaded despite wear on the worm-gear threads and downward shifting of the anti-backlash nut. The worm gear has an integral upwardly extending sleeve portion, the lower portion of which is internally splined and the upper portion of which is internally threaded. The upper portion of the sleeve is thinner than the lower. Both portions of the sleeve are spaced radially from the load stem and an annular space is formed therebetween. Threaded on the load stem above the worm gear and within the annular space formed by the sleeve is an anti-backlash nut, the lower portion of which is externally splined to the worm-gear sleeve. There is an annular space between the upper portion of the anti-backlash nut and the internally threaded upper portion of the worm-gear sleeve into which is received a take-up nut which is externally threaded and in mesh with the upper portion of the worm-gear sleeve. An annular cap is screwed into the housing and bears down on an annular upper bearing which is outside the worm-gear sleeve. The lower race of this bearing bears against a shoulder on the worm gear which bears against the upper race of an annular lower bearing which is supported in the housing.

United States Patent [1 1 Denkowski 1March 20, 1973 [54] ANTI-BACKLASHSCREW JACK [75] Inventor: Walter J. Denkowski, King of Prussia, Pa.

[73] Assignee: Philadelphia Gear Corporation,

King of Prussia, Pa.

[22] Filed: Oct. 7, 1971 [21] Appl. No.: 187,325

[52] US. Cl ..74/441 [51] Int. Cl ..F16h 55/18 [58] Field of Search....74/441 [56] References Cited UNITED STATES PATENTS 2,498,897 2/1950Riedel ..74/44l 2,959,064 11/1960 Geyer et al.

3,479,897 11/1969 l-lolthofer "74/441 Primary Examiner-Leonard l-l.Gerin Attorneyl-lenry N. Paul, Jr. et a1;

[5 7 ABSTRACT An anti-backlash screw jack has a worm gear which is vsupported by upper and lower annular bearings both of which remain fullyloaded despite wear on the worm-gear threads and downward shifting ofthe antibacklash nut. The worm gear has an integral upwardly extendingsleeve portion, the lower portion of which is internally splined and theupper portion of which is internally threaded. The upper portion of thesleeve is thinner than the lower. Both portions of the sleeve are spacedradially from the load stem and an annular space is formed therebetween.Threaded on the load stem above the worm gear and within the annularspace fon'ned by the sleeve is an anti-backlash nut, the lower portionof which is externally splined to the worm-gear sleeve. There is anannular space between the upper portion of the anti-backlash nut and theinternally threaded upper portion of the worm-gear sleeve into which isreceived a take-up nut which is externally threaded and in mesh with theupper portion of the worm-gear sleeve. An annular cap is screwed intothe housing and bears down on an annular upper bearing which is outsidethe worm-gear sleeve. The lower race of this bearing bears against ashoulder on the worm gear which bears against the upper race of anannular lower bearing which is supported in the housing.

4 Claims, 2 Drawing Figures PATENTEDHARZO ms 3.721. 133' INVENTOR. 10 TIWalter J. Denkowski BY Z M *M ATTOR NEYS ANTI-BACKLASH SCREW JACKBACKGROUND OF THE INVENTION The present application relates to ananti-backlash screw jack of the general type shown in McMullen U.S. Pat.No. 3,323,777.

In the McMullen patent, the worm gear is connected to an anti-backlashnut by an annular array of spaced vertical driving pins. Th'e drivenanti-backlash nut is supported by an annular upper bearing which is helddown by a cap screwed into the housing. As wear occurs on the threads ofthe worm gear and the load stem gradually drops, the anti-backlash nutis carried downwardly, being guided by the driving pins. This downwardmovement of the anti-backlash nut unloads the annular bearings, untilthe cap screw is tightened.

SUMMARY OF THE INVENTION A principal object of the present invention isto provide an improved anti-backlash screw jack in which the bearingsremain fully loaded as the anti-backlash nut drops gradually downwardlyas the worm-gear threads wear down.

A further object is to provide an anti-backlash screw jack which avoidsthe use of driving pins as the coupling means for connecting the wormgear to the anti-backlash nut.

Another object is to provide an anti-backlash screw jack havingcalibrated markings thereon to inform the maintenance man of the extentto which the threads of the worm gear have been Worn down, therebygiving the maintenance man advance notice as to when replacement of theworm gear will be necessary.

The foregoing objects are accomplished, in accordance with the presentinvention, by providing a worm gear supported by annular upper and lowerbearings held down by an annular cap screwed or bolted into the housing.The worm gear is provided with an upwardly extending integral sleeveportion which is spaced radially from the load stem, formingtherebetween an annular space. The upper portion of the sleeve isinternally threaded, and is thinner than the lower portion, so that theupper portion of the annular space is wider than the lower. Receivedwithin the annular space is an anti-backlash nut threaded onto the loadstem above the worm gear and separated from the worm gear by a wear gapof preselected dimension. The lower portion of the anti-backlash nut isexternally splined and in engagement with the internal splines of thelower portion of the worm gear sleeve. Between the upper portion of theanti-backlash nut and the internally threaded upper portion of theworm-gear sleeve is an annular space which receives an annular take-upnut which is externally threaded. The lower end of this take-up nut isadapted to bear against an annular shoulder on the anti-backlash nut. Aswear occurs on the threads of the worm gear, the load stem graduallydrops, carrying. with it the anti-backlash nut. This downward movementof the anti-backlash nut is guided by its external splines. There is notunloading effect on the annular bearings of the worm gear. Thesebearings remain fully loaded whether or not the take-up nut istightened.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view, largelyin section, showing an anti-backlash screw jack embodying the presentinvention.

FIG. 2 is a plan view of the screw jack of FIG. 1, taken at section 2-2of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 a screw jackis shown having a housing which supports an externally threaded loadstem 10 driven by a worm gear 20 which is driven by a worm 15. The lowerportion 21 of worm gear 20 is threaded and meshes with threads of loadstem 10. This lower portion 21 functions as the drive portion of theworm gear. An annular lower bearing 61 is supported on base 71 ofhousing 70 for supporting worm gear 20 at an external shoulder 23.

In accordance with the present invention, above the drive portion 21worm gear 20 is provided with an in-,

tegral sleeve portion 22 which is spaced radially from the threads ofload stem 10. In the annular space between the sleeve portion 22 of wormgear 20, an antibacklash nut 30 is provided in mesh with stem 10. Thelower end of anti-backlash nut 30 is initially spaced above an internalshoulder 24 of worm gear 20 by a I selected distance, forming a wear gap34. Wear gap 34 allows the anti-backlash nut 30 to move downwardly withthe load stem 10 relative to the worm gear 20 as the upper surface ofthe threads W of the worm gear 20 wear away.

The lower portion 31 of anti-backlash nut 30 is externally splined andmates with internal splines on the lower portion of sleeve portion 22 ofworm gear 20. The upper portion 32 of the anti-backlash nut 30 isexternally smooth and makes a sliding fit with the internally smoothsurface of an annular take-up nut 40 which occupies the annular spacebetween the antibacklash nut 30 and the sleeve portion 22 of the wormgear 20. Take-up nut 40 is externally threaded and in mesh with theinternal threads on the upper portion of sleeve portion 22 of worm gear20. Anti-backlash nut 30 has a central radial projection forming ashoulder 33 against which the lower end of take-up nut 40 is adapted tobear.

An annular cap 60 is threaded or bolted into the housing 70, and anupper annular bearing 62 is provided between the undersurface of the cap60 and an external shoulder 25 of worm gear 20. It will be noted thatthe annular bearing 62 is outside the worm-gear sleeve portion 22 andthat downward movement of the anti-backlash nut 30 into wear gap 34 doesnot unload bearing 62.

The width of wear gap 34 in the axial direction of stem 10 is made equalto the maximum wear to be permitted on the threads of worm gear 20. Forexample, the width of wear gap 34 may be 0.093 inches. The upper endsurface of sleeve portion 22 of worm gear 20 may preferably be providedwith radial markings calibrated to correspond to the movement in inchesof the anti-backlash nut 30 downwardly into wear gap 34 when the take-upnut 40 is turned downward in the internal threads of sleeve portion 22.An arrow mark provided on the top of take-up nut 40 indicates the amountof downward movement of the anti-backlash nut 30. A segmental lockingplate 50, best seen in FIG. 2, is provided, the undersurface of which isserrated to correspond to the serrations on the upper surface of takeupnut 40. Locking plate 50 is secured to sleeve portion 22 as by screws51.

OPERATION In the course of normal use of the screw jack, the uppersurface W of the threads of worm gear 20 tends to wear away gradually.As this wear occurs, load stem gradually drops, relative to worm gear20, thereby creating space between the upper surface of the threads ofstem 10 and the lower surface of the threads of worm gear 20.

When load stem 10 moves downwardly, due to wear on the threads of wormgear 20, as just described, stem 10 carries down with it theanti-backlash nut 30. In this downwardly movement, anti-backlash nut 30is guided by the splines of its splined connections to the sleeveportion 22 of worm gear 20.

Until the maintenance man tightens the take-up nut 40, a backlashcondition exists in the screw jack. However, as previously noted, thereis no unloading effect on the angular bearing 62. Bearings 61 and 62remain fully loaded, before take-up nut 40 is tightened. Tighteningtake-up nut 40, by turning it rotationally in sleeve portion 22, causesthe lower end of take-up nut 40 to bear against the shoulder 33 ofanti-backlash nut 30. This eliminates the backlash condition, since loadstem 10 is no longer able to move upwardly relative to worm gear despitethe existence of wear space between the upper surface of the threads ofload stem 10 and the under surface of the threads of worm gear 20.

After the maintenance man has initially tightened take-up nut 40, aswear takes place at the upper surface of the threads of worm gear 20, abacklash condition is created. Thus, further tightening of take-up nut40 becomes necessary to eliminate the backlash condition. Tightening oftake-up nut 40, from time to time, continues until anti-backlash nut hasmoved downwardly through the entire distance of wear gap 34, makingimpossible further downward movement of the anti-backlash nut.

In order that the maintenance man may have advance indication as to theextent of wear on the threads of worm gear 20, as evidenced by thedownward movement of the stem 10 and anti-backlash nut 30, the annularupper end surface of sleeve portion 22 of worm gear 20 may be providedwith radial markings calibrated to read in terms of inches of wear gap.In a typical case, the wear gap 34 may have a length of .093 inches inthe axial direction of stem 10 and the annular upper end surface ofsleeve portion 22 may be calibrated accordingly.

As wear on the threads of worm gear 20 occurs, the maintenance workmanwill, from time to time, unscrew and remove locking plate 50 and rotatetake-up nut in a direction to move the take-up nut downwardly until itslower end bears against the shoulder 33 of the anti-backlash nut 30,thereby to prevent lifting of the anti-backlash nut by the stem 10 uponreversal of direction of drive.

Adjustment of the take-up nut 40 is made from time to time until thewear gap 34 has been fully closed by the anti-backlash nut. The factthat the wear gap 34 is approaching being fully closed is communicatedto the maintenance man by a reading of the calibrated markings on theupper end of sleeve portion 22 of worm gear 20. Thus, the maintenanceman is able to anticipate the necessary replacement of the worm gear 20and to place an order for a replacement sufficiently in advance to avoiddelay.

What is claimed is:

1. An anti-backlash screw jack comprising:

a. a housing;

b. an externally threaded load screw;

0. a worm gear having an internally threaded drive portion in mesh withsaid load screw; said worm gear having an integral sleeve portionextending upwardly from said drive portion and spaced radially from saidload screw forming therebetween an annular space, the upper portion ofsaid sleeve portion being internally threaded;

e. an anti-backlash nut in said annular space in mesh with said loadstem and spaced above said drive portion of said worm gear by a maximumwear p f. means connecting the lower portion of said antibacklash nut tosaid worm-gear sleeve portion while allowing for relative slidingmovement therebetween in the axial direction of said load stem;

g. an annular take-up nut in the annular space between the upper portionof said anti-backlash nut and the upper portion of said sleeve portionof said worm gear;

. said take-up nut being externally threaded for engagement with theinternally threaded upper portion of said sleeve portion of said wormgear;

. said anti-backlash nut having a shoulder against which the lower endof said back-up nut is adapted to bear when tightened;

j. means for driving said worm gear rotationally relative to said stem,thereby to also drive rotationally said sleeve portion and saidanti-backlash nut;

k. said take-up nut being adapted to be turned 'manually relative tosaid sleeve portion to impress an axial thrust on said anti-backlashnut;

1. upper and lower annular bearing means supporting said worm gear insaid housing against thrust forces in both axial directions, the loadingon said bearing means remaining substantially unchanged despite axialmovement of said anti-backlash nut relative to said worm gear due towear on said worm-gear threads.

2. Apparatus according to claim 1 characterized in that:

a. said means connecting said lower portion of said anti-backlash nut tosaid worm-gear sleeve portion comprises external splines on saidanti-backlash nut and internal splines on said worm-gear sleeve.

3. Apparatus according to claim 2 characterized in that:

a. an annular cap is provided threadable into the housing of said screwjack for maintaining said upper annular bearing in direct engagementwith said worm gear.

4. Apparatus according to claim 3 characterized in that the upper end ofsaid worm-gear sleeve portion is provided with radial markingscalibrated to correspond to the movement of said anti-backlash nut intosaid wear gap. 5

1. An anti-backlash screw jack comprising: a. a housing; b. anexternally threaded load screw; c. a worm gear having an internallythreaded drive portion in mesh with said load screw; d. said worm gearhaving an integral sleeve portion extending upwardly from said driveportion and spaced radially from said load screw forming therebetween anannular space, the upper portion of said sleeve portion being internallythreaded; e. an anti-backlash nut in said annular space in mesh withsaid load stem and spaced above said drive portion of said worm gear bya maximum wear gap; f. means connecting the lower portion of saidanti-backlash nut to said worm-gear sleeve portion while allowing forrelative sliding movement therebetween in the axial direction of saidload stem; g. an annular take-up nut in the annular space between theupper portion of said anti-backlash nut and the upper portion of saidsleeve portion of said worm gear; h. said take-up nut being externallythreaded for engagement with the internally threaded upper portion ofsaid sleeve portion of said worm gear; i. said anti-backlash nut havinga shoulder against which the lower end of said back-up nut is adapted tobear when tightened; j. means for driving said worm gear rotationallyrelative to said stem, thereby to also drive rotationally said sleeveportion and said anti-backlash nut; k. said take-up nut being adapted tobe turned manually relative to said sleeve portion to impress an axialthrust on said antibacklash nut; l. upper and lower annular bearingmeans supporting said worm gear in said housing against thrust forces inboth axial directions, the loading on said bearing means remainingsubstantially unchanged despite axial movement of said antibacklash nutrelative to said worm gear due to wear on said worm-gear threads. 2.Apparatus according to claim 1 characterized in that: a. said meansconnecting said lower portion of said anti-backlash nut to saidworm-gear sleeve portion comprises external splines on saidanti-backlash nut and internal splines on said worm-gear sleeve. 3.Apparatus according to claim 2 characterized in that: a. an annular capis provided threadable into the housing of said screw jack formaintaining said upper annular bearing in direct engagement with saidworm gear.
 4. Apparatus according to claim 3 characterized in that theupper end of said worm-gear sleeve portion is provided with radialmarkings calibrated to correspond to the movement of said anti-backlashnut into said wear gap.